El Nino's daily Pacific anomaly reached 3.08 C in September, months before its expected peak

Zeke Hausfather, a climatologist at Berkeley Earth, calls the same week a tie. He put the Nino 3.4 anomaly at 3.07 degrees Celsius, level with 2015-16 ^[14]. Scientific American reported 3.05 C, the figure the climate specialist Jeff Berardelli posted, and called it the highest since 2015 ^[21]. The three published values span 0.03 C, half the 0.06 C margin between the new high and November 2015's 3.02 C ^[27]. They come from different boxes of ocean, different days and different baselines. The September 21 reading was above both the 1991-2020 and the 1982-2010 average ^[29]. Hausfather's figures, published on the Climate Brink dashboard, score the Nino 3.4 box against a 30-year average using NOAA's satellite-and-buoy record ^[16].

NOAA has also changed what it scores. The Relative Oceanic Nino Index measures warming in Nino 3.4 against warming across the whole tropical belt, so that general ocean heating does not inflate the number ^[17]. On that index the 1997-98 event still leads ^[7]. The Climate Prediction Center's latest seasonal forecast suggests that record falls soon ^[8], and AFP reported the same expectation ^[17]. Scientific American noted that on a background-adjusted measure this El Nino is not at an all-time high but is hotter than it has ever been for late September ^[24].

So there are two analogues, and which one applies depends on the impact. For anything keyed to absolute warmth, nothing in the record matches this ^[1]. The 3.08 C does not tell you how hard the remote signals will land, because those are what the relative index was built to represent: NOAA says it better captures the pattern's ability to sway weather globally ^[7]. Those signals arrive later. Johnson said El Nino typically brings rainier conditions to the southern United States in late fall and drier than normal weather to the Pacific Northwest. It also brings warmer than average conditions across the northern United States and Canada, along with drought over northern South America and the Amazon and a weakening Indian monsoon ^[13].

Daniel Swain, a climate scientist at the University of California Agriculture and Natural Resources, said this will probably turn out to be the strongest El Nino event of our lives so far ^[23]. Kim Wood of the University of Arizona said sea surface temperatures have been "unparalleled" for this time of year "for a while" ^[22]. Hausfather, in an email to AFP, said September "is very likely to have the highest monthly anomalies on record, barring something quite unexpected in the coming week" ^[15].

The largest figure in the coverage comes from a model. Researchers at the Climate Impact Lab at the University of Chicago estimate the released ocean heat could add about 1.2 C to global land temperatures over the coming months and cause more than 450,000 additional heat-related deaths by February 2027 ^[9]. Science News reports that estimate without the method or the baseline behind it. On why this event is so strong, Johnson said attribution is "really an unanswered question" ^[10]; Nathan Lenssen of the National Center for Atmospheric Research said the debate is unsettled but that a warmer planet has more energy in its oceans for El Nino to release ^[11]. A recent study of coral skeletons found a "striking amplification" of El Ninos over the past 40 years compared with the preindustrial period ^[20].